The perfect single crystal has ultra-high strength but is often accompanied by catastrophic failures after yielding.This study reveals that nano-lamellar TiAl single crystals alleviate the catastrophic failure due to ...The perfect single crystal has ultra-high strength but is often accompanied by catastrophic failures after yielding.This study reveals that nano-lamellar TiAl single crystals alleviate the catastrophic failure due to a post-yielding dislocation retraction through atomistic simulations and theoretical analyses.This dislocation retraction leads to a retained post-yielding strength of1.03 to 2.33 GPa(about 50%of the yielding strength).It is shown that this dislocation retraction is caused by local stress relaxation and interface-mediated image force.The local stress relaxation is due to successive dislocation nucleation in different slip systems,and the interface-mediated image force is caused by the heterogeneous interface.Based on dislocation theory,this study demonstrates that the size effect also plays a vital role in dislocation retraction.Theoretical modeling shows that the dislocation retraction occurs when the lamellar thickness is less than approximately 12 nm.Additionally,the post-yielding dislocation retraction is more pronounced at higher temperatures,making it more effective in alleviating catastrophic failures.These findings demonstrate a viable option for avoiding catastrophic failure of single crystals through nanoscale-lamellar design.展开更多
Estrogen withdrawal in postmenopausal women increases bone loss and bone fragility in the vertebra. Bone loss with osteoporosis not only reduces bone mineral density (BMD), but actually alters bone quality, which can ...Estrogen withdrawal in postmenopausal women increases bone loss and bone fragility in the vertebra. Bone loss with osteoporosis not only reduces bone mineral density (BMD), but actually alters bone quality, which can be comprehensively represented by bone post-yield behaviors. This study aimed to provide some information as to how osteoporosis induced by estrogen depletion could influence the evolution of post-yield microdamage accumulation and plastic deformation in vertebral bodies. This study also tried to reveal the part of the mechanisms of how estrogen deficiency-induced osteoporosis would increase the bone fracture risk. A rat bilateral ovariectomy (OVX) model was used to induce osteoporosis. Progressive cyclic compression loading was developed for vertebra testing to elucidate the post-yield behaviors. BMD, bone volume fraction, stiffness degradation, and plastic deformation evolution were compared among rats raised for 5 weeks (ovx5w and sham5w groups) and 35 weeks (ovx35w and sham35w groups) after sham surgery and OVX. The results showed that a higher bone loss in vertebral bodies corresponded to lower stiffness and higher plastic deformation. Thus, osteoporosis could increase the vertebral fracture risk probably through microdamage accumulation and plastic deforming degradation.展开更多
Changes of failure mechanism with increasing confinement,from extensional to shear-dominated failure,are widely observed in the rupture of intact specimens at the laboratory scale and in rock masses.In an analysis pub...Changes of failure mechanism with increasing confinement,from extensional to shear-dominated failure,are widely observed in the rupture of intact specimens at the laboratory scale and in rock masses.In an analysis published in 2018,both unconfined and triaxial compressive tests were conducted to investigate the strength characteristics of 84 specimens of a Utah coal,including the spalling limits,the ratio of apparent unconfined compressive strength to unconfined compressive strength(UCS),the damage characteristics,and the post-yield dilatancy.These mechanical characteristics were found to be strongly anisotropic as a function of the orientation of the cleats relative to the loading direction,defined as the included angle.A total of four different included angles were used in the work performed in 2018.The authors found that the degree of anisotropic strength differed according to the included angle.However,the transition from extensional to shear failure at the given confinements was not clearly identified.In this study,a total of 20 specimens were additionally prepared from the same coal sample used in the previous study and then tested under both unconfined and triaxial compressive conditions.Because the authors already knew the most contrasting cases of the included angles from the previous work using the four included angles,they chose only two of the included angles(0°and 30°)for this study.For the triaxial compressive tests,a greater confining stress than the mean UCS was applied to the specimens in an attempt to identify the brittle-ductile transition of the coal.The new results have been compiled with the previous results in order to re-evaluate the confinement-dependency of the coal behavior.Additionally,the different confining stresses are used as analogs for different width-to-height(W/H)conditions of pillar strength.Although the W/H ratios of the specimens were not directly considered during testing,the equivalent W/H ratios of a pillar as a function of the confining stresses were estimated using an existing empirical solution.According to this relationship,the W/H at which in situ pillar behavior would be expected to transition from brittle to ductile is identified.展开更多
基金supported by the National Key Research and Development Program of China(Grant No.2019YFA0705400)the National Natural Science Foundation of China(Grant No.51535005)+2 种基金the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures(Grant No.MCMS-I-0419K01)the Fundamental Research Funds for the Central Universities(Grant Nos.NJ2020003,and NZ2020001)a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions。
文摘The perfect single crystal has ultra-high strength but is often accompanied by catastrophic failures after yielding.This study reveals that nano-lamellar TiAl single crystals alleviate the catastrophic failure due to a post-yielding dislocation retraction through atomistic simulations and theoretical analyses.This dislocation retraction leads to a retained post-yielding strength of1.03 to 2.33 GPa(about 50%of the yielding strength).It is shown that this dislocation retraction is caused by local stress relaxation and interface-mediated image force.The local stress relaxation is due to successive dislocation nucleation in different slip systems,and the interface-mediated image force is caused by the heterogeneous interface.Based on dislocation theory,this study demonstrates that the size effect also plays a vital role in dislocation retraction.Theoretical modeling shows that the dislocation retraction occurs when the lamellar thickness is less than approximately 12 nm.Additionally,the post-yielding dislocation retraction is more pronounced at higher temperatures,making it more effective in alleviating catastrophic failures.These findings demonstrate a viable option for avoiding catastrophic failure of single crystals through nanoscale-lamellar design.
基金supported by the National Natural Science Foundation of China (Grants 11472017, 11002004)
文摘Estrogen withdrawal in postmenopausal women increases bone loss and bone fragility in the vertebra. Bone loss with osteoporosis not only reduces bone mineral density (BMD), but actually alters bone quality, which can be comprehensively represented by bone post-yield behaviors. This study aimed to provide some information as to how osteoporosis induced by estrogen depletion could influence the evolution of post-yield microdamage accumulation and plastic deformation in vertebral bodies. This study also tried to reveal the part of the mechanisms of how estrogen deficiency-induced osteoporosis would increase the bone fracture risk. A rat bilateral ovariectomy (OVX) model was used to induce osteoporosis. Progressive cyclic compression loading was developed for vertebra testing to elucidate the post-yield behaviors. BMD, bone volume fraction, stiffness degradation, and plastic deformation evolution were compared among rats raised for 5 weeks (ovx5w and sham5w groups) and 35 weeks (ovx35w and sham35w groups) after sham surgery and OVX. The results showed that a higher bone loss in vertebral bodies corresponded to lower stiffness and higher plastic deformation. Thus, osteoporosis could increase the vertebral fracture risk probably through microdamage accumulation and plastic deforming degradation.
文摘Changes of failure mechanism with increasing confinement,from extensional to shear-dominated failure,are widely observed in the rupture of intact specimens at the laboratory scale and in rock masses.In an analysis published in 2018,both unconfined and triaxial compressive tests were conducted to investigate the strength characteristics of 84 specimens of a Utah coal,including the spalling limits,the ratio of apparent unconfined compressive strength to unconfined compressive strength(UCS),the damage characteristics,and the post-yield dilatancy.These mechanical characteristics were found to be strongly anisotropic as a function of the orientation of the cleats relative to the loading direction,defined as the included angle.A total of four different included angles were used in the work performed in 2018.The authors found that the degree of anisotropic strength differed according to the included angle.However,the transition from extensional to shear failure at the given confinements was not clearly identified.In this study,a total of 20 specimens were additionally prepared from the same coal sample used in the previous study and then tested under both unconfined and triaxial compressive conditions.Because the authors already knew the most contrasting cases of the included angles from the previous work using the four included angles,they chose only two of the included angles(0°and 30°)for this study.For the triaxial compressive tests,a greater confining stress than the mean UCS was applied to the specimens in an attempt to identify the brittle-ductile transition of the coal.The new results have been compiled with the previous results in order to re-evaluate the confinement-dependency of the coal behavior.Additionally,the different confining stresses are used as analogs for different width-to-height(W/H)conditions of pillar strength.Although the W/H ratios of the specimens were not directly considered during testing,the equivalent W/H ratios of a pillar as a function of the confining stresses were estimated using an existing empirical solution.According to this relationship,the W/H at which in situ pillar behavior would be expected to transition from brittle to ductile is identified.
